The present invention generally relates to a liquid crystal driver circuit for driving a Liquid Crystal Display (LCD) apparatus, and more particularly, to a liquid crystal driver circuit that drives Thin Film Transistors (TFTs).
In a liquid crystal driver circuit that drives sources electrodes of TFTs (source driver) used in an LCD, digital image data read from a Random Access Memory (RAM) is converted into analog image signals by a Digital-to-Analog Converter (DACs), and the analog image signals are amplified by an operational amplifier and supplied to source lines of TFTs.
FIG. 7 shows a part of a conventional liquid crystal driver circuit used in a liquid crystal display apparatus. FIG. 7 shows a liquid crystal driver circuit including a RAM 10 that temporarily stores red color (R), green color (G) and blue color (B) image data, DACs 21–23 that convert RGB image data for each pixel read out from the RAM 10 into analog image signals, respectively, and operational amplifiers 131–133 that amplify the analog image signals output from the DACs 21–23, respectively.
The analog image signals amplified by the respective operational amplifiers are supplied to a source line 101 of TFTs 111, 121, . . . , a source line 102 of TFTs 112, 122, . . . and a source line 103 of TFTs 113, 123, . . . , respectively, which drive multiple dots of an LCD panel included in the liquid crystal display apparatus. Capacitors C11, C21, . . . , C12, C22, . . . , and C13, C23, . . . , indicate capacities of the respective dots of the LCD panel.
In this manner, the conventional liquid crystal driver circuit includes Digital-to-Analog Converters, and operational amplifiers in the same number as the number of source lines of the liquid crystal display apparatus. Accordingly, power consumption can increase by rest current of the operational amplifiers and an increased chip area.
Japanese laid-open patent application HEI 5-204334 describes a liquid crystal driver circuit with a reduced number of operational amplifiers. This liquid crystal driver circuit is equipped with data retaining devices that retain image data. The liquid crystal driver circuit is also equipped with an amplifier device that switches and amplifies in a time-divided manner data that is retained at the data retaining devices corresponding to multiple signal lines, and outputs the same in a time-divided manner to output lines corresponding to the respective signal lines. However, although this liquid crystal driver circuit can reduced the number of operational amplifiers, the number of DACs cannot be reduced.
Japanese laid-open patent application HEI 11-175042 describes a liquid crystal driver circuit with a reduced number of DACs. In this liquid crystal driver circuit, before converting digital signals into analog signals via decoders, operations of selecting a specified channel by a multiplexer and decoding data of the selected specified channel are repeated. The data are then demultiplexed. However, with this liquid crystal driver circuit, although the number of DACs can be reduced, the number of output buffer sections that correspond to operational amplifiers cannot be reduced. Moreover, in addition to the multiplexer and demultiplexer, a second latch section for retaining image data of multiple channels is required in a succeeding stage of the demultiplexer.
Accordingly, there is a need for liquid crystal driver circuits that can reduce the power consumption and chip areas by reducing the number of DACs and operational amplifiers without adding an extra latch circuit.